1. Field of the Invention
The present invention relates to a molten steel producing method and particularly to a method of storing a high-carbon molten bath in a reservoir furnace and using the stored molten bath to produce a molten steel in a steel producing furnace.
2. Discussion of Related Art
There are two molten-steel producing methods that are widely practiced; one is so-called blast-furnace-converter process in which iron ore and coke are put in a blast furnace so as to be molten and reduced at high temperature and the thus obtained hot metal whose C content is high is transferred to a converter in which oxygen is blown into the hot metal to decarbonize the metal and produce a molten steel; and the other is electric-furnace process in which scrap is molten in an electric furnace so as to produce a molten steel.
In the latter, electric-furnace process, scrap obtained from, e.g., waste cars, and a slag-producing material such as calcium oxide are put in an electric furnace such as an arc furnace, and electric power is applied to the electric furnace to melt the scrap.
Subsequently, usually, oxygen is blown into the molten steel to remove phosphorus and other impurities, and the concentration of carbon of the molten steel is adjusted.
Then, the molten steel is further heated, and the electric furnace is tilted to output a core portion of the molten steel and remove the slag on the molten steel.
In the former, blast-furnace-converter process, since iron ore is used as the starting material (the iron material), a lot of energy is needed to reduce the iron ore in producing hot metal. In addition, a large equipment is needed. Thus, the equipment cost, the maintenance cost, and the running cost are high.
Moreover, in the former process, the operation of the blast furnace is a continuous operation in which hot metal is continuously outputted from the furnace. Thus, it is substantially impossible to produce only a needed amount of hot metal, i.e., molten steel, at only a timing when the hot metal is needed.
In contrast, in the latter, electric-furnace process, since, usually, scrap is used as the iron material, the energy needed to melt the scrap is less than the energy needed when iron ore is used, by an amount needed to reduce the iron ore. In addition, an equipment needed to perform the latter process is simpler. Thus, the equipment cost, the maintenance cost, and the running cost are lower. Moreover, since the latter process is carried out on a batch basis, it is possible to produce, depending upon the economical circumstances, only a needed amount of molten steel, at only a timing when the steel is needed.
Furthermore, the latter process can be carried out in the nighttime when electric power costs low.
Since the cost of the molten-metal producing process using the electric furnace largely depends on the electric-power cost, the cost of the process can be reduced by operating the electric furnace in the nighttime.
However, it is practically difficult to carry out the process using the electric furnace, all in the nighttime, and at least a portion of the process is carried out also in the daytime when the electric power costs high.
In addition, the molten-steel producing method using the electric furnace cannot help using scrap having a certain quality, for the purpose of producing a final product having a certain quality. This is why the cost of production of molten steel according to this method is high.
That is, it is practically impossible to use, as the iron material, lower scrap that contain much impurities or whose impurities may largely change, or use the lower scrap in a much amount in combination with other sorts of scrap.
Moreover, in the molten-steel producing method using the electric furnace, it is desirable to use scale that has been disposed off, because the scale discarded can be utilized and the cost of production of molten steel can be lowered. However, in the conventional molten-steel producing method, the scale cannot be used as the iron material.
The scale essentially consists of iron oxides such as wustite, magnetite, hematite, etc. that are produced on the surfaces of iron or steel, e.g., when iron or steel is subjected to hot rolling or cast iron is subjected to soaking. Usually, the scale is removed from the iron or steel by acid cleaning, cutting, etc., and then it is discarded.
The Fe content of the scale is about 70 to 80 wt %. Therefore, if the scale can be used as an iron material for producing a molten steel, the cost of production of molten steel can be lowered. However, the scale essentially consists of the iron oxides, and the electric furnace that can melt the scale cannot reduce the scale or recover the Fe component. Thus, in the conventional molten-steel producing method using the electric furnace, the scale cannot be used.
It is therefore an object of the present invention to provide a molten-steel producing method that is free of the above-indicated problem.
According to a first feature of the present invention, there is provided a method of producing a molten steel, comprising the steps of putting, in an electric furnace, an iron material and a carbon material, to melt the iron material and the carbon material and thereby produce a high-carbon molten iron whose carbon content is not lower than 1%, storing, in a reservoir furnace whose capacity is larger than a capacity of the electric furnace, an amount of the high-carbon molten iron that corresponds to a plurality of charges of the electric furnace, and using a portion of the high-carbon molten iron stored in the reservoir furnace, to produce the molten steel in a steel producing furnace.
According to the present invention, an iron material and a carbon material such as breeze or coal are put in an electric furnace, and a high-carbon molten iron whose carbon content is not lower than 1% is produced in the electric furnace. The high-carbon molten iron produced is temporarily stored in a reservoir furnace, and a portion of the high-carbon molten iron stored in the reservoir furnace is taken and used to produce a molten steel in a steel producing furnace.
Thus, according to the present invention, the high-carbon molten iron can be produced in the electric furnace in the nighttime when electric power costs low, so that the molten iron produced may be stored in the reservoir furnace. The high-carbon molten iron stored in the reservoir furnace can be used to produce the molten steel in the steel producing furnace, in the daytime when the electric power costs high.
According to a second feature of the present invention that includes the first feature, the step of using the high-carbon molten iron to produce the molten steel, comprises putting the high-carbon molten iron, and scrap, in the steel producing furnace to produce the molten steel.
According to this feature, when the high-carbon molten iron is used to produce the molten steel in the steel producing furnace, the high-carbon molten iron and another sort of iron material, i.e., scrap are put in the steel producing furnace, and are molten in mixture. In this case, since the latent heat of the high-carbon molten iron, that is, the thermal energy of the molten iron, and the heat of reaction produced when the molten iron is decarbonized and CO and CO2 gases are produced, are effectively utilized, the molten steel can be produced, with reduced energy, in the steel producing furnace.
Since the high-carbon molten iron can be produced in the nighttime when the electric power costs low, the total energy needed to produce the molten steel can be reduced, which contributes to reducing the cost of the electric power needed to produce the molten steel.
The above-indicated advantage results from the present molten-steel producing method including the steps in which the high-carbon molten iron is produced using the electric furnace, is stored in the reservoir furnace, and is used to produce the molten steel in the steel producing furnace.
The reason why the C content of the high-carbon molten iron is not lower than 1% is as follows: If the C content is lower than 1%, then it is substantially impossible to transfer the high-carbon molten iron from the electric furnace to the reservoir furnace and store the molten iron in the reservoir furnace for a certain time.
The melting point of the high-carbon molten iron changes with the C content thereof, such that as the C content increases, the melting point lowers and accordingly the molten iron becomes harder to solidify. Therefore, a storable time in which the molten iron can be stored in the reservoir furnace increases.
Here, the storable time (a storage time including, e.g., respective handling times needed to transfer the high-carbon molten iron from the electric furnace to the reservoir furnace and to transfer the molten iron from the reservoir furnace to the steel producing furnace (e.g., an electric furnace)) needs to be not less than 1 hour, and the present inventors"" studies have elucidated that when the C content is not lower than 1%, the high-carbon molten iron can be stored for a time not less than 1 hour.
This is why the present invention requires that the C content of the high-carbon molten iron be not lower than 1%.
According to the present invention, the temperature of the high-carbon molten iron can be easily controlled because the high-carbon molten iron is molten and produced in the electric furnace. Thus, the high-carbon molten iron can be advantageously outputted at a high temperature.
For example, when a hot metal as a high-carbon molten steel is outputted from a blast furnace, the temperature of the hot metal is about 1,300 to 1,350xc2x0 C. In contrast, according to the present invention, the high-carbon molten iron can be outputted, from the electric furnace, at a high temperature of, e.g., 1,500xc2x0 C.
Since the high-carbon molten iron can be outputted at the high temperature, a storable time in which the molten iron can be stored in the reservoir furnace can be increased.
Thus, according to the present invention, a time when, and an amount in which, a molten steel is produced in the steps in which the high-carbon molten iron is produced using the electric furnace, is stored in the reservoir furnace, and is used to produce the molten steel in the steel producing furnace, can be easily controlled depending upon the economical circumstances.
According to a third feature of the present invention that includes the first or second feature, the steel producing furnace comprises an electric furnace.
According to this feature, when the high-carbon molten iron taken from the reservoir furnace is used to produce the molten steel in the steel producing furnace, an electric furnace can be used as the steel producing furnace.
As described above, the high-carbon molten iron may be mixed, and molten, with scrap in the electric furnace so as to produce a molten steel. The energy needed to produce the molten steel in the electric furnace, i.e., the electric power can be reduced.
However, according to the present invention, the steel producing furnace may be provided by a different sort of furnace than the electric furnace.
For example, a high-carbon molten iron whose C content is about 1.5% may be transferred as a seed bath to an AOD furnace (a steel producing furnace), so that the molten iron is decarbonized and smelted in the furnace to produce a stainless steel.
Since the high-carbon molten iron whose C content is about 1.5% can be stored in the reservoir furnace for about 10 hours, as described later, the high-carbon molten iron can be used, according to the present invention, to produce a stainless steel while enjoying the advantages of the present invention.
The present invention is essentially characterized in that when the high-carbon molten iron taken from the electric furnace is stored in the reservoir furnace, an amount of the high-carbon molten iron that corresponds to a plurality of charges of the electric furnace is simultaneously stored in the reservoir furnace, and a portion of the high-carbon molten iron stored in the reservoir furnace is used to produce a molten steel in the steel producing furnace.
It is possible to store, in the reservoir furnace, an amount of the high-carbon molten iron that corresponds to just one charge of the electric furnace and use all the high-carbon molten iron stored in the reservoir furnace, to produce a molten steel in the steel producing furnace.
In this case, however, dispersion in respective compositions of the respective charges of high-carbon molten iron, each produced in the electric furnace, directly influence quality of the molten steels produced in the steel producing furnace.
In contrast, according to the present invention, an amount of the high-carbon molten iron that corresponds to a plurality of charges of the electric furnace is simultaneously stored in the reservoir furnace, and accordingly the respective compositions of the respective charges of high-carbon molten iron are averaged in the reservoir furnace.
For example, in the case where an amount of the high-carbon molten iron that corresponds to 8 charges of the electric furnace is stored in the reservoir furnace, the respective compositions of the 8 charges of high-carbon molten iron are averaged in the reservoir furnace and the dispersion in those compositions is leveled off.
Thus, when a portion of the high-carbon molten iron stored in the reservoir furnace is outputted, the composition of the portion outputted is equal to the averaged composition.
Therefore, according to the present invention, it is possible to use lower scrap that has the problem that respective compositions of different batches thereof largely differ from each other and accordingly cannot be used in the conventional methods, or to use the lower scrap in a greater proportion in combination with one or more different sorts of iron materials.
According to a fourth feature of the present invention that includes any of the first to third features, the step of putting the iron material and the carbon material to produce the high-carbon molten iron, comprises putting scrap as the iron material.
According to this feature, when a high-carbon molten iron is produced using the electric furnace, scrap can be used. More specifically described, lower scrap that has the problem that impurities of one batch thereof largely differ from those of another batch thereof can be used, or the lower scrap can be used in a greater proportion in combination with one or more different sorts of iron material. In addition, when a molten steel is produced in the steel producing furnace in the final step, the lower scrap can be used as an iron material, or can be used in a greater proportion in combination with one or more different sorts of iron material.
Thus, according to this feature, the cost of production of molten steel can be lowered while the quality of the molten steel produced is maintained at a high level.
According to a fifth feature of the present invention that includes the fourth feature, the step of putting the iron material and the carbon material to produce the high-carbon molten iron, comprises putting scrap and scale as the iron material.
According to this feature, when a high-carbon molten iron is produced using the electric furnace, scale can be used together with scrap.
That is, scale that has been disposed off can be used as a material for producing steel, which contributes to lowering the cost of the materials needed to produce steel.
Since, in the high-carbon-molten-iron producing process using the electric furnace, the carbon material is input together with the iron material, the scale as the iron oxides can be reduced by the carbon material, and accordingly the Fe component can be efficiently recovered. This is another advantage of the present invention.